It is conventional to secure gas turbine blades to the disc of a gas turbine with dove tail fir tree grooves in the disc. A fir tree root on the blade engages these grooves. Precise location of the blade in the radially outward direction is established by precise locations on the two fir trees. Therefore it is designed to bear against the support surface with the blade in it's radially outermost position. Inboard clearances are of course required to permit insertion of the blade.
In such an arrangement some means are required to axially retain the blade at its desired position.
At high rpm's centrifugal force will establish the blade in its outer position. However it is required that the blade have substantially the same position at balancing speed (1000 rpm) and also at tip grinding speed (100 rpm).
Sealing is required to deter gas passage from the gas path upstream of the blade, between blade platforms, to the space under the blade at the downstream side thereof.
Damping of the blades is also a benefit to reduce vibratory stresses of blades during operation.